Fix semantic on flash messages

[Stef-Rousset]

🎩 What? Why?

This PR adds semantic p tag in flash messages.

• it updates the announcement_cell.rb file methods to add p tags when it is relevant.
  The issue has been spotted in the audit of Angers city (page 82)
• it also adds p tag in the flash_helper_extension

📌 Related Issues

Link your PR to an issue

• Related to https://github.com/orgs/OpenSourcePolitics/projects/26/views/1?pane=issue&itemId=133478303&issue=OpenSourcePolitics%7Cintern-tasks%7C178

Testing

• log in to the application and see that the text of the flash message is displayed in a p tag (screenshot one)
• go to the show page of an evaluating proposal and see that the flash messages are displayed in a p tag, and that there is not empty p tags before and after the div class="rich-text-display" (screenshot two)

📷 Screenshots

ONE

TWO

♥️ Thank you!